More than 30 women were raped in a coordinated attack in the Democratic Republic of Congo on New Year's Day, the aid agency Médecins Sans Frontières has reported.

MSF said 33 women were raped in Fizi, South Kivu, in the eastern part of the war-torn country.

"Women had been restrained with ropes or beaten unconscious with the butt of a gun before being attacked, some in front of their children," said Annemarie Loof, an official with the medical aid organisation. "Up to four armed men were involved at a time and homes and shops were looted," she said in a statement.

MSF teams treated 14 women at the hospital in Fizi on 3 January, and 19 the next day. Two severely wounded people were transferred to Baraka hospital, one with serious head injuries after being beaten with a rock, the other having been shot in the chest.

The United Nations says rape is a frequent weapon of war in Congo. Around 15,000 women were raped in eastern Congo in 2009. It is believed that many cases go unreported.

It confirmed the rape of at least 240 people between 30 July and 4 August in the Walikale region of Congo's North Kivu province by Rwandan and Congolese rebels in a brazen attack near a UN peacekeepers' base.

The reported victims in previous attacks ranged from a month-old baby boy to a 110-year-old woman.

Survivors have accused the FDLR rebel group – which is led by perpetrators of the Rwandan genocide who fled to Congo – along with Congolese Mai-Mai militia.

Between 200 and 400 rebels systematically raped and looted in the town of Luvungi and five nearby villages, according to the International Medical Corps.

It said husbands were forced to watch their wives being raped, while others were dragged into a nearby forest.

The attacks occurred in parts of the country where armed rebel groups moved into areas considered to be pro-government but lacking in army or police protection.

Médecins Sans Frontières said there was no sign that the security situation was improving.

"MSF is extremely concerned about the current situation in and around Fizi," Loof said yesterday. "People are fleeing the area fearing further violent attacks."

The agency provided medical and psycho-social care for 5,600 rape victims in North and South Kivu in 2009.